//判断屏幕尺寸
function Rem() {
    var docEl = document.documentElement,
        oSize = docEl.clientWidth / 7.5;
    if(oSize > 85) {
        oSize = 85; //  限制rem值   640 / 7.5 ≈ 85
    }

    docEl.style.fontSize = oSize + 'px';
}
window.addEventListener('resize', Rem, false);
Rem();
//判断屏幕尺寸End